In recent years, attention has been paid to titanium oxide particles in the field of photocatalysts. For example, Patent Documents 1 and 2, and Non Patent Documents 1 and 2 disclose titanium oxide particles which have a decahedral box-like shape and are mainly formed of anatase-type crystals (hereinafter referred to as “decahedral titanium oxide particles”).
Patent Documents 1 and 2, and Non Patent Document 1 describe that since decahedral titanium oxide particles have large surface areas per unit mass and have high crystallinity with fewer internal defects, the titanium oxide particles have high activity as photocatalysts. Furthermore, Non Patent Document 2 describes that decahedral titanium oxide particles have a high ratio of the highly reactive (001) plane and are promising as photocatalysts.
A method for producing titanium oxide particles includes, for example, a method in which a hydrothermal reaction using hydrofluoric acid described in Non Patent Document 2 is utilized. However, this production method involves a construction using hydrofluoric acid, and is therefore unsuited for the production on an industrial scale.
The methods for producing decahedral titanium oxide particles described in Patent Documents 1 and 2, and Non Patent Document 1 are methods in which titanium tetrachloride (TiCl4) vapor and oxygen (O2) gas are introduced into a reaction tube, and then these gasses are heated from the outside of the reaction tube, and thus titanium oxide particles (TiO2) are produced by a reaction represented by the following reaction scheme (1):TiCl4+O2→TiO2+2Cl2  (1)
When the production methods are used, a powder product containing titanium oxide particles can be obtained in the downstream side of the reaction tube. This powder product contains a large proportion of decahedral titanium oxide particles.
However, decahedral titanium oxide particles obtained by the production methods have photocatalytic activity which is insufficient for use in a practical photocatalytic product, and thus further improvement in photocatalytic activity has been required.